1. Field of the Invention
The present invention relates to a method of automatically selecting a BNC/D-sub signal in a DPMS (Display Power Management Signalling) display device. More particularly, the present invention relates to a method of and circuit for automatically selecting a signal without using a separate signal selecting switch and for displaying the content of the signal by way of OSD (On Screen Display).
2. Discussion of Related Art
As one of peripheral equipment used in a computer, a display device processes signals transmitted from the computer to create an image so as for the users to recognize it visually.
FIG. 1 is a schematic view of the internal circuit of such a display device.
As shown in FIG. 1, the display device is composed of: a video card 10 built in a computer (not shown) in order to supply video signals R, G and B and horizontal/vertical sync signals H/V-Sync which are needed to produce an image; a microcomputer 20 receptive to the horizontal/vertical sync signals H/V-Sync from the video card 10 and generating a screen control signal to control the screen of a monitor; horizontal and vertical deflection circuits 40 and 30 for providing beam deflection in response to the horizontal/vertical sync signals H/V-Sync, respectively, in such a manner that the electron beams generated from electron guns in CRT 80 are deflected by means of a DY (Deflection Yoke) from left top to right bottom of the screen so as to create an image like a picture; a high voltage circuit 50 for supplying a high voltage to the anode of the CRT 80 with a feedback pulse applied from the output of the horizontal deflection circuit 40 using the principle of a switching circuit and the high voltage technique; a video pre-amplifier 60 functioning as a low voltage amplifier to boost weak video signals R, G and B sent from the video card 10 to a specified high level; and a video main amplifier 70 for boosting the developed video signals R, G and B from the video pre-amplifier 60 to 40-60 V.sub.pp so as to supply energy to the respective pixels.
Recently, such a display device employs DPMS, which is to restrict a normal operation by interrupting part of power supply with a view to preventing surplus power consumption, that is, saving power when the device does not receive a designated synchronous signal from the computer.
Functions associated with DPMS are performed as several modes, i.e. the normal mode, stand-by mode, suspend mode and off mode.
To dilate upon these functions, the system enters the normal in response to both horizontal/vertical sync signals transferred from the computer, the stand-by mode when no horizontal sync signal is received, the suspend mode without a vertical sync signal, and the off mode when either of the horizontal/vertical sync signals is received.
There are two methods for transmitting horizontal/vertical sync signals from the computer or other data processing devices to the display device.
The one method is to transmit the signals through a single cable containing both video and synchronous signal lines, as illustrated in FIG. 2a. The signals applied on the respective pins of the connector may be varied somewhat depending on the manufacturer of the display device.
The other employs a cable which is an exclusive means for transmitting the horizontal/vertical sync signals, provided separately from a cable for the video signals.
D-sub signals are used to execute OS programs because they are weak in high frequency characteristics, and BNC signals are usually employed for the programs that require high resolution in the high frequency band, such as CAD.
There have been used two methods for selecting a BNC or D-sub signal.
The one method is called a switch selection method to manually select the BNC or D-sub signal with a separate convert switch irrespective of the signal applied to the display device from the computer.
The other is an improved switch selection method wherein the signal applied from a main body is recognized to conduct an automatic switching operation making use of a relay in selecting a BNC or D-sub signal.
However, the aforementioned conventional methods arise several problems as follows:
First, the user himself has to take the trouble to operate the switch manually in the switch selection method. This will deteriorate the working efficiency and productivity in using a switch during a separate step of checking the BNC/D-sub signal.
Second, the relay switching method takes some time in discriminating the type of a signal applied from the main body and causes noises as well as failures due to an increase in the contact resistance arising from the relay's switching operation for a long time.
Third, it is hard to select keys in the DPMS operation because it would encounter some problems in displaying an OSD under no horizontal/vertical sync signals.